A. Technical Field
The present invention relates to a surface-crosslinking process for a water-absorbent resin, particularly, a surface-crosslinking process of a water-absorbent resin to obtain a water-absorbing agent which displays fast absorption speed and excellent absorption capacity under a load (a water-absorbent resin which has specific or larger values of properties).
In addition, the present invention relates to: a water-absorbing agent which contains only a small quantity. of fine partides and is excellent in absorption capacity under a load, liquid-permeability under a load, and impact resistance; and a production process for such a water-absorbing agent.
B. Background Art
In recent years, water-absorbent resins which display water absorption from tens of times to hundreds of times its own weight have been developed, and a variety of water-absorbent resins are used for the purposes which need water absorption or water retention, for example, in sanitary material fields such as sanitary articles and paper diapers, or in agricultural and horticultural fields, or in food fields such as freshness retention, or in industrial fields such as condensation prevention and coldness retention materials.
Known examples of such a water-absorbent resin include:
hydrolysates of starch-acrylonitrile graft polymers (JP-B-49-043395); neutralization products of starch-acrylic acid graft polymers (JP-A-51-125468); saponification products of vinyl acetate-acrylic acid ester copolymers (JP-A-52-014689); hydrolysates of acrylonitrile copolymers or acrylamide copolymers (JP-B-53-015959), or their crosslinked matters; self-crosslinking type sodium polyacrylates as obtained by reversed-phase suspension polymerization (JP-A-53-046389); and crosslinked matters of partially neutralized polyacrylic acids (JP-A-55-084304).
The abilities that are demanded of water-absorbent resins are different according to the purposes for which they will be used, but examples of properties as desired of water-absorbent resins for sanitary materials include: high absorption capacity under a load, fast absorption speed, and high liquid-permeability, upon contact with aqueous liquids. However, relations between these properties do not necessarily display positive correlations, so it was difficult to improve these properties simultaneously.
Among the above properties, two properties of absorption speed and absorption capacity under a load are desired of the water-absorbent resin as fundamental properties. Thus, the following materials are, for example, proposed: sanitary materials using a water-absorbent resin of high water absorption speed and high absorption capacity under a load in a high concentration of 60 weight % or more of its core (U.S. Pat. No. 5,149,335), and a water-absorbent resin which exhibits a high absorption capacity of 12 g/g or more under a load of 60 g/cm.sup.2 and has high water absorption speed (U.S. Pat. No. 5,712,316, EP 0707603).
Then, as attempts to enhance the absorption speed of the water-absorbent resin, attempts are for example made to decrease the particle diameter of the water-absorbent resin, or to granulate the water-absorbent resin, or to form the water-absorbent resin into scales, for the purpose of enlarging the surface area of the water-absorbent resin. However, when the water-absorbent resin is formed into a small particle diameter, the water-absorbent resin forms so-called "fisheyes" due to contact with aqueous liquids, so that the absorption speed is decelerated rather than accelerated. In addition, when the water-absorbent resin is granulated, each of the resultant granules themselves falls into a state of "fisheyes" due to contact with aqueous liquids, so that the absorption speed is decelerated rather than accelerated. In addition, when the water-absorbent resin is formed into scales, its absorption speed is improved, but is still insufficient because gel-blocking is induced, and further, forming the water-absorbent resin into scales is uneconomical in that the resultant water-absorbent resin is necessarily bulky and therefore needs larger facilities for transportation and storage.
Thus, as means other than means for improving the surface area of the above water-absorbent resin, there are also some proposed arts in which molecular chains in the neighborhood of the surface of the water-absorbent resin are crosslinked to raise the crosslinking density of the surface layer, namely, the formation of "fisheyes" is prevented by surface-crosslinking to raise the absorption speed. In addition, such surface-crosslinking is especially important for raising the absorption capacity under a load of the water-absorbent resin.
Such arts are, for example, disclosed in JP-A-57-044627, JP-A-58-042602, JP-B-60-018690, JP-A-58-180233, JP-A-59-062665, JP-61-016903, U.S. Pat. No. 5,422,405, U.S. Pat. No. 5,597,873, U.S. Pat. No. 5,409,771, EP 450923, EP 450924, EP 668080. Furthermore, there is a known process in which the granulation of water-absorbent resin is carried out simultaneously with its surface-crosslinking for the purpose of attaining the surface-crosslinking which improves the water absorption speed (WO 91/17200, Publication of Internal Patent Application as entered the national phase in Japan (Kohyo) No. 06-216042, and U.S. Pat. No. 5,002,986, U.S. Pat. No. 5,122,544, U.S. Pat. No. 5,486,569, EP 695763). In addition, there is also a known art in which the particle size is kept constant during surface-crosslinking (all examples of preferred embodiments as set forth in JP-A-58-042602). Furthermore, there is also a known art in which a crosslinking agent is added to a hydrogel, and the resultant mixture is dried and then divided finely and then further crosslinked (U.S. Pat. No. 5,145,906, U.S. Pat. No. 5,385,983, U.S. Pat. No. 5,447,727, U.S. Pat. No. 563,316).
Indeed the water absorption speed of the water-absorbent resin may be improved to some extent by the above surface-crosslinking, but it is actually necessary to enlarge the specific surface area of the resultant water-absorbent resin for the purpose of obtaining a water-absorbent resin having high water absorption speed, because the water absorption speed of the water-absorbent resin, fundamentally, greatly depends on the contact area with liquids to be absorbed.
Thus, there is a proposed art in which a foamed water-absorbent resin is further surface-crosslinked (Publication of Internal Patent Application as entered the national phase in Japan (Kohyo) No. 08-509521, and JP-A-05-237378, JP-A-63-088410, WO 96/17884, U.S. Pat. No. 5,314,420, U.S. Pat. No. 5,399,591, U.S. Pat. No. 5,451,613, U.S. Pat. No. 5,462,972, EP 574435, EP 707603, EP 744435). In addition, there is also a known art in which the average particle diameter is finely controlled.
However, when the water absorption speed is raised by enlarging the specific surface area of the water-absorbent resin (to be surface-crosslinked) by foaming the resin or decreasing its average particle diameter to the fine size, the water-absorbent resin (to be surface-crosslinked) absorbs the crosslinking agent (as added for surface-crosslinking) in a moment, so that it is difficult to uniformly coat the surface of the water-absorbent resin with the surface-crosslinking agent. Therefore, generally, because the water-absorbent resin of a large specific surface area displays too fast absorption speed, it is difficult to uniformly surface-crosslink such a water-absorbent resin, so the resultant absorption capacity under a load is low.
Furthermore, the above control of the average particle diameter further causes problems of fine particles. That is to say, it is generally preferable that the amount of fine particles with a particle diameter less than 150 .mu.m in the water-absorbent resin is as low as possible, in view of liquid-permeability, dust-generatability, and workability, and further in view of properties for absorbent articles. However, the fine control of the average particle diameter to increase the specific surface area results in formation of a large amount of fine particles as by-products, and accompanying this increase of fine particles, the properties of the water-absorbent resin deteriorates or the cost rises due to recovery of fine particles. Furthermore, industrial fine control of the particle diameter is difficult because of stability of the particle size, and results in scattering of the properties of the water-absorbent resin such as absorption capacity under load and water absorption speed.
That is to say, the two properties of water absorption speed and absorption capacity under a load, which are the most fundamental properties of the water-absorbent resin, conflict, because as the specific surface area of the water-absorbent resin increases, the uniform surface-crosslinking gets more difficult